Adsorption of Hydrogen on Dispersed Copper−Rhodium Bimetallic Crystallites

Supported bimetallic Rh−Cu/A1 2 O 3 samples of different X Cu [ N Cu /( N Cu + N Rh ) atomic ratios] were prepared by impregnating a 3.8 wt % Rh/A1 2 O 3 sample with different amounts of Cu(NO 3 ) 2 solution and characterized with techniques of hydrogen chemisorption and 2 H NMR spectroscopy. The irreversible hydrogen uptake of the bimetallic samples, measured from the chemisorption, increases upon impregnating a small amount of copper but decreases as the X Cu becomes larger than 0.1. Deuterium atoms adsorbed on the bimetallic samples exhibit only a single 2 H NMR peak. However, both the line width (Δυ) and the chemical shift (δ) of this peak vary significantly with the X Cu ratio. Phenomena observed from chemisorption and NMR spectroscopy may be explained satisfactorily by the formation of two alloy phases, i.e., a rhodium-rich phase [(Rh), X Cu ≈ 0.05] and a copper-rich phase, [(Cu), X Cu ≈ 0.8], on the surface of bimetallic crystallites. Detected variations in the Δυ and the δ suggest a fast chemical exchange of the deuterium atoms adsorbed on the surfaces of these two alloy phases. The rate constant of this exchange process is estimated from the variation of Δυ to be k ex = (1.4 ± 0.6) × 10 6 exp(−8.6 ± 0.8 kJ mol -1 /( RT )) s -1 . From the average X Cu calculated from the δ of 2 H NMR and the X Cu of the bulk composition estimated from ICP-MS, a surface enrichment of copper on supported Cu−Rh crystallites was indicated.